1. Technical Field
The present invention relates to a pellicle for lithography for use as a dustproof cover for a photomask, a reticle or the like in the manufacture of a semiconductor device such as a large-scale integrated circuit, a very-large-scale integrated circuit (LSI) or the like or a liquid crystal display panel and a method for manufacturing the pellicle for lithography.
2. Description of the Related Art
In the case of manufacturing a semiconductor device such as a large-scale integrated circuit (LSI), a very-large-scale integrated circuit or the like, or a liquid crystal display panel, a semiconductor wafer or a mother substrate for a liquid crystal display panel is irradiated with exposure light via an exposure stencil such as a photomask or a reticle (hereinafter collectively referred to as “photomask” in this specification), whereby a pattern of the photomask is transferred onto the semiconductor wafer or the mother substrate for a liquid crystal display panel so that a pattern of the semiconductor device or the liquid crystal display panel is formed.
Therefore, when the semiconductor wafer or the liquid crystal display panel mother substrate is irradiated with exposure light via the photomask, any foreign materials such as dust particles stick to the photomask, reflect or absorb the exposure light, so that not only does the pattern transferred onto the semiconductor wafer or the liquid crystal display panel mother substrate become deformed and the edge portion of the pattern become blurred but the underlying surface is also blackened by soiling, whereby the size, quality, appearance and the like of the semiconductor wafer or the liquid crystal display panel mother substrate are degraded. As a result, the pattern of the photomask cannot be transferred onto the semiconductor wafer or the liquid crystal display panel mother substrate in the desired manner and the performance of the semiconductor wafer or the liquid crystal display panel mother substrate is lowered, whereby the yield ratio of the semiconductor wafer or the liquid crystal display panel mother substrate is inevitably reduced.
In order to solve these problems, the operation of exposing the semiconductor wafer or the liquid crystal display panel mother substrate is generally conducted in a clean room. However, even in such a case, since it is extremely difficult to completely prevent foreign materials from attaching to the surface of the photomask, the operation of exposing the semiconductor wafer or the liquid crystal display panel mother substrate is usually conducted with a dust proof cover, called a pellicle, having a high transmittance with respect to the exposure light mounted on the surface of the photomask.
Generally, a pellicle is manufactured by adhering a pellicle membrane made of a material having a high transmittance with respect to exposure light, such as a cellulose based resin like nitrocellulose or cellulose acetate, fluoride resin or the like, to one surface of a pellicle frame made of aluminum, stainless steel, polyethylene or the like by applying a good solvent for the material of the pellicle membrane onto the one surface of the pellicle frame and adhering the air-dried pellicle membrane onto the one surface of the pellicle frame, or adhering the pellicle membrane onto the one surface of the pellicle frame using an adhesive agent such as acrylic resin, epoxy resin, fluorine resin or the like, and then forming an agglutinant layer composed of polybutene resin, polyvinyl acetate resin, acrylic resin, silicone resin or the like and adapted for adhering the photomask to the pellicle frame on the opposite surface of the pellicle frame, and providing a release layer or a separator for protecting the agglutinant layer on the agglutinant layer (See Japanese Patent Application Laid Open No. 58-219023, U.S. Pat. No. 4,861,402, Japanese Patent Publication No. 63-27707 and Japanese Patent Application Laid Open No. 7-168345, for example).
In the case of mounting the thus constituted pellicle on the surface of the photomask and exposing the semiconductor wafer or the liquid crystal display panel mother substrate via the photomask, since foreign materials such as dust particles attach to the surface of the pellicle and do not directly attach to the surface of the photomask, it is possible to avoid the effect of foreign materials such as dust particles by projecting exposure light in such a manner that the light is focused on the pattern formed on the photomask.
Further, in order to reduce the difference in pressure between that in a closed space defined by a pellicle mounted on a photomask and that outside of the pellicle, a part of the pellicle frame may be formed with a small vent hole through which the difference in pressure is canceled and a filter may be provided for preventing foreign materials contained in air moving through the small vent hole from entering the closed space defined by the pellicle and the photomask.
Since the pellicle membrane is generally formed of a thin resin sheet, in order to support the pellicle membrane by the pellicle frame in such a manner that the pellicle membrane does not sag, it is necessary to adhere the pellicle membrane to the pellicle frame while a predetermined tensile force is being applied to the pellicle membrane.
As a result, in the case of using the commonly adopted pellicle having a rectangular cross section, the tensile force applied to the pellicle membrane inevitably deforms the pellicle frame to bow inward after the pellicle membrane has been adhered to a pellicle frame.
This inward deformation of the pellicle frame becomes particularly pronounced in a large-size pellicle including a pellicle frame having a long side length and generally used for fabricating a printed circuit board, a liquid crystal display or the like.
On the other hand, in order to reduce the manufacturing cost of a photomask, the exposure area of the photomask is required to be as large as possible, and since the exposure area decreases when the pellicle frame is deformed inward, the inward deformation of the pellicle frame must be reduced as much as possible.
These problems can be technically solved by increasing the rigidity of the pellicle frame by means of increasing its cross sectional area, for example. However, the actual situation is that the area inside the pellicle frame has to be determined by the need to ensure an exposure area that is as large as possible and that it is necessary to ensure a space for handling a photomask so as to fix or transport it outside of the pellicle frame. Thus, each of the sides of the pellicle frame has a linear shape determined so as to meet the above described requirements.
Further, it has been proposed to reduce the inward deformation of the pellicle frame by forming the pellicle frame of a material having a higher rigidity. In fact, it has been found that in the case of forming the pellicle frame of carbon fiber reinforced plastic (CFRP) or titanium instead of aluminum alloy, the inward deformation of the pellicle frame can be made smaller than in the case of forming the pellicle frame of aluminum alloy. However, since these materials are not only expensive but also hard to process, the cost of the pellicle increases, so that this solution is impractical.
In order to reduce the inward deformation of a pellicle frame, Japanese Patent No. 4,286,194 proposes forming a pellicle frame in such a manner that each of at least one pair of facing sides thereof has a central portion with an arcuate shape projecting outward, re-entrant portions positioned on opposite sides of the central portion each having a re-entrant arcuate shape projecting inward, and linear shaped portions at the opposite sides of the re-entrant portions, so that after the pellicle membrane is adhered to the pellicle frame, the pellicle frame assumes a desired shape owing to the attachment of the pellicle membrane to the pellicle frame.
According to this method, the shape of the pellicle frame after adherence of the pellicle membrane thereto can be controlled in the desired manner, but this method is not suitable for mass-producing the pellicle.
More specifically, in order to ensure that the shape of the pellicle frame after adherence of the pellicle membrane thereto can be controlled in the desired manner, it is necessary to design the shape of the pellicle frame with consideration to the tensile force distribution in the pellicle membrane. However, the tensile force distributions in mass-produced pellicle membranes are not constant and inevitably vary to some extent. As a result, in the case where the tensile force in the pellicle membrane is larger than the design tensile force, the pellicle frame after adherence of the pellicle membrane is excessively inwardly deformed, while in the case where the tensile force in the pellicle membrane is smaller than the design tensile force, the side of the pellicle frame remains deformed outwardly even after adherence of the pellicle membrane in the case of a pellicle frame formed in the manner proposed by Japanese Patent No. 4,286,194 wherein each of at least one pair of facing sides thereof has the central portion having the arcuate shape projecting outward, the re-entrant portions positioned on the opposite sides of the central portion each having the re-entrant arcuate shape projecting inward and the linear shaped portions at the opposite sides of the re-entrant portions. Generally, the tolerance of the inner dimension of the pellicle frame is determined to be on the safe side with respect to the pattern region of the photomask, taking into account the deformation of the pellicle frame caused by the tensile force of the pellicle membrane, the misplacement of the pellicle to the photomask, and other such factors. However, since the pellicle frame can be deformed outward only due to mechanical deformation thereof, the outward deformation of the pellicle frame is not considered when the pellicle frame is being designed. Therefore, in the case where the side of the pellicle frame projects outwardly owing to the tensile force of the pellicle membrane applied to the pellicle frame being smaller than the design tensile force of the pellicle membrane, there is some risk of the pellicle frame interfering with a pellicle adhering device for adhering the pellicle to a photomask, a transporting device in a photomask exposure device or the like when the pellicle is handled.
For example, in the typical pellicle adhering device disclosed in Japanese Patent No. 4,354,789, the pellicle is set with one of the long sides of the pellicle frame directed downward. Therefore, in the case where one of the long sides of the pellicle frame projects outwardly, the one of the long sides of the pellicle frame projecting outwardly is deformed inwardly when the pellicle is set in the pellicle adhering device, so that there is a considerable risk of the pellicle membrane being wrinkled.